The present disclosure relates to positively chargeable single-layer electrophotographic photosensitive members each including a photosensitive layer that contains a hole transport material and two or more electron transport materials selected from a group consisting of compounds each having a particular chemical structure. The present disclosure also relates to image forming apparatuses that includes such a positively chargeable single-layer electrophotographic photosensitive member as an image bearing member.
An electrophotographic image forming apparatus includes an electrophotographic photosensitive member. Examples of an electrophotographic photosensitive member include inorganic photosensitive members and organic photosensitive members. An inorganic photosensitive member includes a photosensitive layer made from an inorganic material, such as selenium or amorphous silicon. An organic photosensitive member includes a photosensitive layer mainly made from organic materials, such as a binder resin, a charge generating material, and a charge transport material. Of these electrophotographic photosensitive members, organic photosensitive members are widely used for the following reason. That is, organic photosensitive members can be produced more easily than inorganic photosensitive members, and materials for the photosensitive layer can be selected from a wide variety of materials. Organic photosensitive members thus provide high design flexibility.
Examples of such organic photosensitive members include single-layer organic photosensitive members and multi-layer organic photosensitive members. A single-layer organic photosensitive member includes a photosensitive layer containing both a charge generating material and a charge transport material within the layer. A multi-layer organic photosensitive member includes a photosensitive layer that is a stack of a charge generating layer containing a charge generating material and a charge transport layer containing a charge transport material. As compared with multi-layer organic photosensitive members, single-layer organic photosensitive members are known to be simple in configuration, easy to be manufactured, and capable of reducing occurrence of film defects.
With the use of such an electrophotographic photosensitive member, an image forming process including the following steps (1) through (5) is performed.
(1) charging a surface of the electrophotographic photosensitive member;
(2) exposing the charged surface of the electrophotographic photosensitive member to light to form an electrostatic latent image;
(3) developing the electrostatic latent image with toner in the presence of a developing bias voltage applied;
(4) transferring the thus formed toner image to a transfer target by reversal development; and
(5) heating to fix the toner image transferred to the transfer target.
The electrophotographic photosensitive member is rotated for use during such an image forming process. Therefore, a phenomenon occurs that the potential (light potential) of a portion which has been exposed during image formation remains, and therefore, even after the charging step in the next turn of the photosensitive member, a desired charge potential (dark potential) cannot be obtained at the portion which has been exposed in the previous turn. This phenomenon is called “transfer memory.” Portions with and without transfer memory have different image densities, and therefore, it is difficult to obtain a satisfactory image.
Furthermore, single-layer electrophotographic photosensitive members may be of a positively chargeable type and a negatively chargeable type. The techniques employed for charging the electrophotographic photosensitive member include contact charging and non-contact charging. The use of a positively chargeable single-layer electrophotographic photosensitive member is preferable, and the combined use of a positively chargeable single-layer electrophotographic photosensitive member with a contact-type charger is more preferable for the following reason. That is, the surface of an electrophotographic photosensitive member can be charged substantially without generating oxidizing gas such as ozone, which adversely affects the life of the electrophotographic photosensitive member or the office environment. However, the combined use of a positively chargeable single-layer electrophotographic photosensitive member with a contact-type charger presents a problem of being particularly prone to transfer memory.
In view of the above circumstances, demand exists for positively chargeable single-layer electrophotographic photosensitive members capable of reducing occurrence of transfer memory during image formation. The use of a charge transport material having an excellent charge transport function is effective to reduce occurrence of transfer memory. Examples of a charge transport material having an excellent charge transport function include a compound usable as a charge transport material and represented by the following chemical formula:
